WO2021039266A1

WO2021039266A1 - Austenitic heat-resistant steel

Info

Publication number: WO2021039266A1
Application number: PCT/JP2020/029240
Authority: WO
Inventors: 平田　弘征; 吉澤　満; 淳一樋口; 克樹田中
Original assignee: 日本製鉄株式会社
Priority date: 2019-08-29
Filing date: 2020-07-30
Publication date: 2021-03-04
Also published as: JPWO2021039266A1; JP7265203B2; WO2021039266A9; US20220325394A1; KR20220034226A; EP4023776A1; EP4023776A4; CN114341381A

Abstract

This austenitic heat-resistant steel has a chemical composition containing, in mass%, 0.04-0.12% of C, 0.10-0.30% of Si, 0.20-0.80% of Mn, 0-0.030% of P, 0.0001-0.0020% of S, 0.0005-0.0230% of Sn, 2.3-3.8% of Cu, 0.90-2.40% of Co, 22.0-28.0% of Ni, 20.0-25.0% of Cr, 0.01-0.40% of Mo, 2.8-4.2% of W, 0.20-0.80% of Nb, 0.0010-0.0050% of B, and 0.16-0.30% of N, and also optionally containing at least one among Al, O, V, Ti, Ta, C, Mg, and an REM, with the remainder comprising Fe and impurities, wherein 0.0012%≤[%S]+[%Sn]≤2.5X[%B]+0.0125% is satisfied.

Description

Austenitic heat resistant steel

The present invention relates to austenitic heat resistant steel.
The present application claims priority based on Japanese Patent Application No. 2019-156592 filed in Japan on August 29, 2019, the contents of which are incorporated herein by reference.

In recent years, from the viewpoint of reducing the environmental load, the operating conditions of boilers for power generation have been increasing in temperature and pressure on a global scale, and the materials used for superheater tubes and reheater tubes are superior. It is required to have high temperature strength and corrosion resistance.
As a material satisfying such a requirement, a large amount of N and Ni are contained in order to increase the high temperature strength, and more than 20% of Cr is contained in order to enhance the corrosion resistance and the steam oxidation resistance at high temperature. Various austenitic heat resistant steels are disclosed.

For example, Patent Document 1 contains 20% to 27% of Cr, 22.5% to 32% of Ni, and 0.1% to 0.3% of N to provide high temperature strength, steam oxidation resistance, and resistance to steam oxidation. Heat-resistant austenitic stainless steel with enhanced fireside corrosiveness and tissue stability has been proposed.
Patent Document 2 contains austenitic stainless steel having excellent high-temperature strength and creep ductility containing Cr of more than 22% to less than 30%, Ni of more than 18% to less than 25%, and N of 0.1% to 0.35%. Steel has been proposed.
Patent Document 3 contains Cr of more than 22% to less than 30%, Ni of more than 18% to less than 25%, N of 0.1% to 0.35%, and the amount of impurity elements such as Sn and Sb. Austenitic heat-resistant steels that are excellent in high-temperature strength and workability after long-term use by reducing the amount have been proposed.
Patent Document 4 contains 15% to 30% Cr, 6% to 30% Ni, 0.03% to 0.35% N, and reduces impurity elements such as P, S, and Sn. , Austenitic stainless steels having excellent high temperature strength and embrittlement cracking resistance of welded parts during long-term use have been proposed.

By the way, it is necessary to periodically stop the boiler for power generation to check its soundness, and at that time, the temperature of the members such as the pipes used will drop. The above-mentioned austenitic stainless steels and heat-resistant steels have excellent high-temperature strength, and although they have excellent performance for the problems to be solved by each, when the weldability is not sufficient and / or, It was found that when the temperature drops after long-term use at high temperature, sufficient toughness may not be stably obtained.

Japan Special Table 2002-537486 Japanese Patent Application Laid-Open No. 2004-250783 Japanese Patent Application Laid-Open No. 2009-84606 International Publication No. 2009/044796

The present invention has been made in view of the above situation. An object of the present invention is to provide an austenitic heat-resistant steel having excellent weldability, excellent creep strength, and stable toughness after being held at a high temperature for a long time.

In order to solve the above-mentioned problems, the present inventors have Cr as 20.0% to 25.0%, Ni as 22.0% to 28.0%, and Co as 0.90% in terms of creep strength. It contains ~ 2.40%, N is 0.16% ~ 0.30%, and from the viewpoint of weldability (ability to form back beads at the time of first layer welding), S: 0.0001% ~ 0. A detailed investigation was carried out on the toughness of austenitic heat-resistant steels essentially containing 0020% and Sn: 0.0005% to 0.0230% after holding (heating) at high temperature for a long time. As a result, the following findings were clarified.
(A) The toughness of steel held at high temperature for a long time decreases remarkably as the contents of S and Sn increase. As a result of observing the fracture surface after the impact test, as the content of S and Sn increased, the proportion of the region broken at the austenite grain boundary increased, and S and Sn were detected on the fracture surface. From this result, the reason why the toughness of the steel containing S and Sn decreases after long-term holding at high temperature is that S and Sn contained in the steel segregate at the austenite grain boundaries during long-term holding at high temperature. It is presumed that this element reduces the binding force of the grain boundaries.
(B) On the other hand, as a result of the studies by the present inventors, in order to secure the toughness after holding at a high temperature for a long time, S and Sn should be reduced as much as possible within the range where the weldability is not hindered, and the total content thereof should be increased. Therefore, it was found that it is effective to contain B in an appropriate range. The reason for this is that B contained in the steel has a high diffusion rate and segregates into the austenite grain boundaries faster than S and Sn when held at high temperature for a long time, and as a result, B has a decrease in grain boundary bonding force due to S and Sn. It is considered that this is to suppress the decrease in toughness and reduce the decrease in toughness.

The present invention has been completed based on the above findings, and the gist thereof is in the austenitic heat-resistant steel shown below.
(1) The austenite-based heat-resistant steel according to one aspect of the present invention has a chemical composition of mass%, C: 0.04% to 0.12%, Si: 0.10% to 0.30%, Mn: 0.20% to 0.80%, P: 0% to 0.030%, S: 0.0001% to 0.0020%, Sn: 0.0005% to 0.0230%, Cu: 2.3% ~ 3.8%, Co: 0.90% ~ 2.40%, Ni: 22.0% ~ 28.0%, Cr: 20.0% ~ 25.0%, Mo: 0.01% ~ 0 .40%, W: 2.8% to 4.2%, Nb: 0.20% to 0.80%, B: 0.0010% to 0.0050%, N: 0.16% to 0.30 %, Al: 0% to 0.030%, O: 0% to 0.030%, V: 0% to 0.08%, Ti: 0% to 0.08%, Ta: 0% to 0.08 %, Ca: 0% to 0.010%, Mg: 0% to 0.010%, REM: 0% to 0.080%, the balance is composed of Fe and impurities, and the formula (i) is satisfied. Austenite heat-resistant steel.
0.0012% ≤ [% S] + [% Sn] ≤ 2.5 x [% B] + 0.0125% (i)
However, [% S], [% Sn], and [% B] in the formula (i) indicate the contents of S, Sn, and B in mass%, respectively.
(2) The austenitic heat-resistant steel according to (1) above has a chemical composition of V: 0.01% to 0.08%, Ti: 0.01% to 0.08%, Ta: 0.01. One or more selected from% to 0.08%, Ca: 0.001% to 0.010%, Mg: 0.001% to 0.010%, REM: 0.0005% to 0.080%. May be contained.

According to the above aspect of the present invention, an austenitic stainless steel having excellent weldability and having both stable toughness after holding at a high temperature for a long time (for example, 500 hours or more at 450 to 800 ° C.) and excellent creep strength. Heat resistant steel can be provided. The austenite-based heat-resistant steel according to the above aspect of the present invention can be used for a long time at high temperature, for example, in a boiler pipe of a coal-fired power plant, an oil-fired power plant, a waste incineration power plant, a biomass power plant, or a decomposition pipe in a petrochemical plant. Suitable for the equipment used.

It is a figure which shows the groove shape at the time of a welding test.

Hereinafter, the austenitic heat-resistant steel according to the embodiment of the present invention (austenitic heat-resistant steel according to the present embodiment) will be described. The austenitic heat-resistant steel according to the present embodiment is, for example, a steel compatible with the austenitic stainless steel and the austenitic heat-resistant steel described in JIS G0203: 2009.

<Chemical composition>
The austenitic heat-resistant steel according to this embodiment has a predetermined chemical composition. The reasons for limiting its chemical composition are as follows.
In the following description, the "%" indication of the content of each element means "mass%". Further, in the present specification, the numerical range represented by using "-" means a range including the numerical values before and after "-" as the lower limit value and the upper limit value unless otherwise specified.

C: 0.04% to 0.12%
C is an element that stabilizes the austenite structure and combines with Cr to form carbides, improving the creep strength at high temperatures. In order to obtain this effect sufficiently, the C content needs to be 0.04% or more. The C content is preferably 0.05% or more, more preferably 0.06% or more.
On the other hand, when C is excessively contained, a large amount of carbide is precipitated and the toughness is lowered. Therefore, the C content is set to 0.12% or less. The C content is preferably 0.11% or less, more preferably 0.10% or less.

Si: 0.10% to 0.30%
Si has a deoxidizing effect and is an element necessary for ensuring corrosion resistance and oxidation resistance at high temperatures. In order to obtain the effect, the Si content needs to be 0.10% or more. The Si content is preferably 0.12% or more, more preferably 0.15% or more.
On the other hand, when Si is excessively contained, the stability of the austenite structure is lowered and the creep strength is lowered. Therefore, the Si content is set to 0.30% or less. The Si content is preferably 0.28% or less, more preferably 0.25% or less.

Mn: 0.20% to 0.80%
Like Si, Mn is an element having a deoxidizing effect. It is also an element that stabilizes the austenite structure and contributes to the improvement of creep strength. In order to obtain these effects, the Mn content needs to be 0.20% or more. The Mn content is preferably 0.25% or more, more preferably 0.30% or more.
On the other hand, when the Mn content becomes excessive, the creep ductility decreases. Therefore, the Mn content is set to 0.80% or less. The Mn content is preferably 0.75% or less, more preferably 0.70% or less.

P: 0% to 0.030%
P is contained as an impurity and is an element that enhances the susceptibility to liquefaction cracking during welding. Further, when P is contained in a large amount, the creep ductility also decreases. Therefore, an upper limit is set for the P content so that the P content is 0.030% or less. The P content is preferably 0.028% or less, more preferably 0.025% or less. The P content is preferably reduced as much as possible, that is, the content may be 0%. However, an extreme reduction in P content leads to an increase in steelmaking costs. Therefore, the preferable lower limit of the P content is 0.001%, and the more preferable lower limit is 0.002%.

S: 0.0001% to 0.0020%
S is an element that segregates at the austenite grain boundaries during retention at high temperatures and weakens its binding force. Therefore, if the S content is high, the toughness of the heat-resistant steel after being held at a high temperature for a long time decreases. In the content range of other elements in the austenitic heat-resistant steel according to the present embodiment, in order to prevent a decrease in toughness, the S content must be 0.0020% or less, and the Sn content and B content. It is necessary to satisfy the relationship between the quantity and the following. The S content is preferably 0.0018% or less, more preferably 0.0015% or less. From the viewpoint of toughness, the S content is preferably reduced as much as possible. However, S is also an element that affects the flow of hot water in the molten pool during welding, increases the penetration depth, and enhances welding workability, particularly back wave forming ability during first layer welding. Therefore, it is necessary to set the S content to 0.0001% or more and satisfy the relationship with Sn, which will be described later. The S content is preferably 0.0002% or more, more preferably 0.0003% or more.

Sn: 0.0005% -0.0230%
Sn is an element that evaporates from the molten pool during welding and contributes to the formation of an energizing path for the arc, and also has the effect of improving the weldability by increasing the penetration depth. In order to obtain this effect, the Sn content is 0.0005% or more in the content range of other elements in the austenitic heat-resistant steel according to the present embodiment, and the relationship with the S content described later is satisfied. There is a need to. The Sn content is preferably 0.0010% or more, more preferably 0.0015% or more.
On the other hand, when Sn is excessively contained, Sn segregates at the austenite grain boundaries during holding at a high temperature, weakening the binding force of the grain boundaries. As a result, the toughness of steel after being held at high temperature for a long time is reduced. Therefore, in the content range of other elements in the austenitic heat-resistant steel according to the present embodiment, it is necessary to set the Sn content to 0.0230% or less and to satisfy the relationship between the S content and the B content described later. is there. The Sn content is preferably 0.0220% or less, more preferably 0.0200% or less.

Cu: 2.3% to 3.8%
Cu is an element that enhances the stability of the austenite structure and finely precipitates during holding at high temperatures, contributing to the improvement of creep strength. In order to obtain this effect sufficiently, it is necessary to set the Cu content to 2.3% or more. The Cu content is preferably 2.5% or more, more preferably 2.7% or more.
On the other hand, if Cu is excessively contained, the hot workability is lowered. Therefore, the Cu content is set to 3.8% or less. The Cu content is preferably 3.5% or less, more preferably 3.3% or less.

Co: 0.90% to 2.40%
Co is also an element that enhances the stability of the austenite structure and contributes to the improvement of creep strength. In order to obtain the effect sufficiently, the Co content needs to be 0.90% or more. The Co content is preferably 1.00% or more, more preferably 1.20% or more, and even more preferably 1.40% or more.
On the other hand, when Co is excessively contained, not only the effect is saturated, but also Co is a very expensive element, which causes an increase in cost. Therefore, the Co content is set to 2.40% or less. The Co content is preferably 2.20% or less, more preferably 2.00% or less.

Ni: 22.0% to 28.0%
Ni is an element that enhances the stability of the austenite structure and contributes to the improvement of creep strength. In order to obtain the effect sufficiently, the Ni content needs to be 22.0% or more. The Ni content is preferably 22.2% or more, more preferably 22.5% or more.
On the other hand, since Ni is a very expensive element, if Ni is excessively contained, not only the effect is saturated but also the cost is increased. Therefore, Ni is set to 28.0% or less. The Ni content is preferably 27.8% or less, more preferably 27.5% or less.

Cr: 20.0% to 25.0%
Cr is an element effective for ensuring oxidation resistance and corrosion resistance at high temperatures. In addition, Cr is an element that forms fine carbides and contributes to the improvement of creep strength. In order to obtain these effects sufficiently, it is necessary to set the Cr content to 20.0% or more. The Cr content is preferably 20.5% or more, more preferably 21.0% or more.
On the other hand, when Cr is excessively contained, the stability of the austenite structure is lowered and the creep strength is lowered. Therefore, the Cr content is set to 25.0% or less. The Cr content is preferably 24.5% or less, more preferably 24.0% or less.

Mo: 0.01% to 0.40%
Mo is an element that dissolves in steel and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain the effect sufficiently, the Mo content needs to be 0.01% or more. The Mo content is preferably 0.02% or more, more preferably 0.03% or more.
On the other hand, when Mo is excessively contained, the stability of the austenite structure is remarkably lowered, and the creep strength is lowered. Furthermore, since Mo is an expensive element, excessive content leads to an increase in cost. Therefore, the Mo content is set to 0.40% or less. The Mo content is preferably 0.38% or less, more preferably 0.35% or less.

W: 2.8% to 4.2%
W is an element that dissolves in steel and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain the effect sufficiently, the W content needs to be 2.8% or more. The W content is preferably 3.0% or more, more preferably 3.2% or more.
On the other hand, when W is excessively contained, the stability of the austenite structure is lowered, and the creep strength is rather lowered. Therefore, the W content is set to 4.2% or less. The W content is preferably 4.0% or less, more preferably 3.8% or less.

Nb: 0.20% to 0.80%
Nb is an element that precipitates in the grains of austenite as fine carbides and nitrides and contributes to the improvement of creep strength and tensile strength at high temperatures. In order to obtain the effect sufficiently, the Nb content needs to be 0.20% or more. The Nb content is preferably 0.25% or more, more preferably 0.30% or more.
On the other hand, when Nb is excessively contained, a large amount of carbonitride is precipitated and the creep ductility is lowered. Therefore, the Nb content is set to 0.80% or less. The Nb content is preferably 0.75% or less, more preferably 0.70% or less.

B: 0.0010% to 0.0050%
B improves the creep strength by finely dispersing the grain boundary carbides, and segregates at the grain boundaries during holding at high temperature to suppress the segregation of S and Sn at the grain boundaries after holding at high temperature. It is an element that contributes to improving the toughness of steel. In order to sufficiently obtain these effects, it is necessary to set the B content to 0.0010% or more and satisfy the relationship between the S content and the Sn content described later. The B content is preferably 0.0012% or more, more preferably 0.0015% or more.
On the other hand, when B is excessively contained, the crack sensitivity of the heat-affected zone during welding increases. Therefore, the B content is set to 0.0050% or less. The B content is preferably 0.0048% or less, more preferably 0.0045% or less.

N: 0.16% to 0.30%
N is an element that stabilizes the austenite structure and dissolves in steel or precipitates as a nitride, which contributes to the improvement of high-temperature strength. In order to obtain the effect sufficiently, the N content needs to be 0.16% or more. The N content is preferably 0.18% or more, more preferably 0.20% or more.
On the other hand, if N is excessively contained, the ductility is lowered. Therefore, the N content is set to 0.30% or less. The N content is preferably 0.28% or less, and more preferably 0.26% or less.

Al: 0% to 0.030%
Al is an element added as an antacid. However, when Al is excessively contained, the cleanliness of the steel deteriorates and the hot workability deteriorates. Therefore, the Al content needs to be 0.030% or less. The Al content is preferably 0.025% or less, more preferably 0.020% or less. The lower limit does not need to be set in particular, that is, the Al content may be 0%. However, an extreme reduction in Al content leads to an increase in manufacturing cost. Therefore, the Al content is preferably 0.001% or more, more preferably 0.002% or more.

O: 0% to 0.030%
O (oxygen) is an element contained as an impurity. When O is excessively contained, the hot workability is lowered and the ductility is deteriorated. Therefore, the O content needs to be 0.030% or less. The O content is preferably 0.025% or less, more preferably 0.020% or less. The lower limit does not need to be set in particular, that is, the O content may be 0%. However, an extreme reduction in O content leads to an increase in manufacturing cost. Therefore, the O content is preferably 0.001% or more, more preferably 0.002% or more.

0.0012% ≤ [% S] + [% Sn] ≤ 2.5 x [% B] + 0.0125% (1)
In the austenitic heat-resistant steel according to the present embodiment, it is necessary to control the content of each element as described above, and further, the S content, the Sn content, and the B content need to satisfy the above formula (1). is there.
Here, [% S], [% Sn], and [% B] in the formula (1) indicate the contents of S, Sn, and B in mass%, respectively.
S and Sn are elements that segregate at the austenite grain boundaries during holding at high temperatures and weaken their binding force. Therefore, in general, steel containing S and Sn has a reduced toughness after being held at a high temperature for a long time. However, as the present inventors have found, B has a high diffusion rate and segregates at the austenite grain boundaries faster than S and Sn, and suppresses a decrease in toughness due to segregation of S and Sn at the grain boundaries. .. In order to obtain the effect sufficiently, it is necessary to make the total content of S and Sn equal to or less than 2.5 × [% B] + 0.0125% with respect to the content of B.
On the other hand, the lower the content of S and Sn, the more advantageous it is to improve the toughness after holding at high temperature, but they affect the convection and arc phenomenon of the molten pool during welding, respectively. It has the effect of increasing the penetration depth and improving the weldability (particularly, the back wave forming ability at the time of initial layer welding). In the austenitic heat-resistant steel according to the present embodiment, the total amount of S and Sn needs to be 0.0012% or more in order to obtain the effect. The total content of preferred S and Sn is 0.0015% or more, and a more preferable total content is 0.0018% or more.

The austenitic heat-resistant steel according to the present embodiment basically contains the above elements and the balance is Fe and impurities. In addition to the above, the following group is used instead of a part of Fe as an alloy component. It may contain at least one element in the above. However, since these elements do not necessarily have to be contained, the lower limit is 0%. The reasons for the limitation are described below.

V: 0 to 0.08%
V is an element that combines with carbon (C) or nitrogen (N) to form fine carbides or carbonitrides and contributes to the improvement of creep strength. Therefore, it may be contained as needed. When this effect is obtained, the V content is preferably 0.01% or more, more preferably 0.02% or more.
However, when V is excessively contained, a large amount of carbonitride is precipitated and the creep ductility is lowered. Therefore, even when it is contained, the V content needs to be 0.08% or less. The V content is preferably 0.07% or less, more preferably 0.06% or less. Even more preferably, it is 0.04% or less.

Ti: 0 to 0.08%
Like V, Ti is an element that combines with carbon or nitrogen to form fine carbides or carbonitrides and contributes to the improvement of creep strength. Therefore, it may be contained as needed. When this effect is obtained, the Ti content is preferably 0.01% or more, more preferably 0.02% or more.
However, when Ti is excessively contained, a large amount of carbonitride is precipitated and the creep ductility is lowered. Therefore, even when it is contained, the Ti content needs to be 0.08% or less. The Ti content is preferably 0.07% or less, more preferably 0.06% or less.

Ta: 0-0.08%
Like V and Ti, Ta is an element that combines with carbon or nitrogen to form fine carbides or carbonitrides and contributes to the improvement of creep strength. Therefore, it may be contained as needed. When this effect is obtained, the Ta content is preferably 0.01% or more, more preferably 0.02% or more.
However, when Ta is excessively contained, a large amount of carbonitride is precipitated and the creep ductility is lowered. Therefore, even when it is contained, the Ta content needs to be 0.08% or less. The Ta content is preferably 0.07% or less, more preferably 0.06% or less.

Ca: 0 to 0.010%
Ca is an element having an effect of improving hot workability during production. Therefore, it may be contained as needed. When this effect is obtained, the Ca content is preferably 0.001% or more, more preferably 0.002% or more.
However, when Ca is excessively contained, it combines with oxygen (O) and the cleanliness is remarkably lowered, so that the hot workability is deteriorated. Therefore, even when it is contained, the Ca content is 0.010% or less. The Ca content is preferably 0.008% or less, more preferably 0.006% or less.

Mg: 0 to 0.010%
Like Ca, Mg is an element that has the effect of improving hot workability during manufacturing. Therefore, it may be contained as needed. When this effect is obtained, the Mg content is preferably 0.001% or more, more preferably 0.002% or more.
However, when Mg is excessively contained, it is combined with oxygen (O), the cleanliness is remarkably lowered, and the hot workability is deteriorated. Therefore, even when it is contained, the Mg content is 0.010% or less. The Mg content is preferably 0.008% or less, more preferably 0.006% or less.

REM: 0 to 0.080%
Like Ca and Mg, REM is an element having an effect of improving hot workability during production. Therefore, it may be contained as needed. When this effect is obtained, the REM content is preferably 0.0005% or more, more preferably 0.001% or more.
However, when REM is excessively contained, it is combined with oxygen, the cleanliness is remarkably lowered, and the hot workability is deteriorated. Therefore, even when it is contained, the REM content is 0.080% or less. The REM content is preferably 0.060% or less, more preferably 0.050% or less.
"REM" is a general term for a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of one or more elements of REM. Further, REM is generally contained in misch metal. Therefore, for example, it may be added in the form of misch metal so that the REM content is within the above range.

[Production method]
The austenitic heat-resistant steel according to the present embodiment is made into a slab by casting, for example, molten steel having the above-mentioned predetermined chemical composition, and the slab is hot-forged, then hot-worked and, if necessary, hot-worked. It is obtained by performing cold working, forming into a predetermined shape, and then performing a solution heat treatment (solution heat treatment) in which the product is held at 1050 to 1280 ° C. for 2 to 60 minutes and then cooled with water. The processing conditions such as hot forging, hot working, and cold working are not particularly limited and may be appropriately determined according to the shape.

The austenitic heat-resistant steel according to this embodiment is used for equipment used at high temperatures, such as a boiler for power generation. Examples of equipment used at high temperatures include boiler pipes for coal-fired power plants, oil-fired power plants, waste incineration power plants and biomass power plants, and decomposition pipes for petrochemical plants.
Here, "use at high temperature" includes, for example, an embodiment of use in an environment of 450 ° C. or higher and 800 ° C. or lower (further, 500 ° C. or higher and 750 ° C. or lower).

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

Hot forging and hot rolling were performed on the ingots in which the materials of reference numerals A to N having the chemical compositions shown in Tables 1A and 1B (the balance is Fe and impurities: the unit is mass%) were melted and cast. It was formed into a plate shape with a thickness of 18 mm.
This plate-shaped material was heated to 1180 ° C., held at that temperature for 30 minutes, and then water-cooled to obtain austenitic heat-resistant steels (Nos. 1 to 14).

[Charpy impact test / evaluation of toughness]
From the austenitic heat-resistant steel after the solution treatment, the front and back surfaces were ground by machining, and a plurality of plate materials (base material for impact test) having a plate thickness of 15 mm, a width of 150 mm, and a length of 150 mm were collected. Further, a part of the impact test base material was subjected to an aging heat treatment at 700 ° C. for 1000 hours.
After that, for the impact test base material that was not subjected to aging heat treatment and the impact test base material that was subjected to aging heat treatment, three 2 mm V notch full-size Charpy impact test pieces with notches were processed from the central part in the thickness direction of each plate. They were collected one by one and subjected to a Charpy impact test.
The Charpy impact test was conducted in accordance with JISZ2242: 2005. The test was carried out at 20 ° C., and those having an average absorbed energy value of 27 J or more for the three test pieces were regarded as "passed", and among them, the individual values of the absorbed energy of the three test pieces were all 27 J or more. The ones that become "excellent" and the others are "acceptable". On the other hand, those in which the average value of absorbed energy of the three test pieces was less than 27 J were regarded as "failed".

[Welding test / Evaluation of weldability]
Further, from the austenitic heat-resistant steel after the solution treatment, the front and back surfaces were ground by machining, and a plate material (base material for welding test) having a plate thickness of 15 mm, a width of 50 mm and a length of 100 mm was collected. After performing the groove processing shown in FIG. 1 in the longitudinal direction of the base metal for welding test, butt welding is performed by automatic gas tungsten arc welding with Ar as the shield gas, and "no filler metal" and "welded filler material". The first layer was welded under the condition of "Yes".
For welding, if there is no filler metal, the heat input is 6 kJ / cm, and if there is a filler metal, JIS-Z3334 (2011) SNi6617 with an outer diameter of 1.2 mm is used as the filler material, and the heat input is 9 kJ / cm. Butt welding was performed as cm.
Welding workability is "passed" when the back bead is formed over the entire length of the weld line of the obtained welded joint, and among them, "excellent" is when the width of the back bead is 2 mm or more over the entire length of the weld line. Is less than 2 mm, but the one with a back bead with a width of 1 mm or more is regarded as "OK", and there is a part of the two joints where the back bead is not formed or the bead width is less than 1 mm. Welding workability was judged to be "failed".

[Creep rupture test / evaluation of creep strength]
In addition, for the austenitic heat-resistant steel that passed the impact test and the welding test, a round bar creep test piece was collected from the impact test base material that had not been subjected to the aging heat treatment, and a creep rupture test was performed. At that time, a creep rupture test was conducted under the condition that the target rupture time of the base metal was 1000 hours at 700 ° C. × 167 MPa. The creep rupture test was performed in accordance with JISZ2271: 2010.
Those having a breaking time exceeding the target breaking time (1000 hours) were regarded as "pass", and those shorter than that were regarded as "failed".

From Table 2, No. 1 produced using the symbols A to E, M, and N satisfying the conditions specified in the present invention. It can be seen that Nos. 1 to 5, 13 and 14 stably obtain excellent toughness after being held at a high temperature for a long time, and also have weldability and creep strength.

On the other hand, No. 1 using the symbols F to H. In 6 to 8, the total content of S and Sn exceeded the range of the relational expression with the B content specified in the present invention. Therefore, the effect of suppressing the decrease in grain boundary binding force due to the segregation of S and Sn grain boundaries by B could not be sufficiently obtained. As a result, the toughness after aging heat treatment (holding at high temperature for a long time) did not satisfy the target.
No. using the symbols I and J. In 9 and 10, since the contents of S and Sn exceeded the upper limit, respectively, the decrease in grain boundary binding force due to the segregation of these elements at the grain boundary became remarkable. As a result, the toughness after aging heat treatment (holding at high temperature for a long time) did not satisfy the target.
In addition, No. In No. 11, since the total content of S and Sn was less than the range specified in the present invention, the effect of improving the back wave forming ability by these elements could not be obtained, and the welding workability was inferior.
No. using the symbol L. In No. 12, the Co content was below the range specified in the present invention. As a result, a sufficient creep strength improving effect could not be obtained.

It can be seen that only when the requirements of the present invention are satisfied as described above, stable and excellent toughness can be obtained after long-term holding without impairing weldability, and sufficient creep strength can also be obtained.

According to the present invention, it is possible to provide an austenitic heat-resistant steel having excellent weldability, stable toughness after being held at a high temperature for a long time, and excellent creep strength.

Claims

The chemical composition is mass%,
C: 0.04% to 0.12%,
Si: 0.10% to 0.30%,
Mn: 0.20% to 0.80%,
P: 0% to 0.030%,
S: 0.0001% to 0.0020%,
Sn: 0.0005% -0.0230%,
Cu: 2.3% to 3.8%,
Co: 0.90% to 2.40%,
Ni: 22.0% to 28.0%,
Cr: 20.0% to 25.0%,
Mo: 0.01% to 0.40%,
W: 2.8% to 4.2%,
Nb: 0.20% to 0.80%,
B: 0.0010% to 0.0050%,
N: 0.16% to 0.30%,
Al: 0% to 0.030%,
O: 0% to 0.030%,
V: 0% to 0.08%,
Ti: 0% to 0.08%,
Ta: 0% to 0.08%,
Ca: 0% to 0.010%,
Mg: 0% to 0.010%,
REM: 0% to 0.080%,
The balance consists of Fe and impurities.
And satisfy equation (1),
Austenitic heat-resistant steel characterized by this.
0.0012% ≤ [% S] + [% Sn] ≤ 2.5 x [% B] + 0.0125% (1)
However, [% S], [% Sn], and [% B] in the formula (1) indicate the contents of S, Sn, and B in mass%, respectively.
The chemical composition
V: 0.01% -0.08%,
Ti: 0.01% -0.08%,
Ta: 0.01% -0.08%,
Ca: 0.001% to 0.010%,
Mg: 0.001% -0.010%,
REM: 0.0005% -0.080%,
Contains one or more selected from,
The austenitic heat-resistant steel according to claim 1.